The act of using a sauce or other such condiment just before passing it to the person who has requested it. The highjacking of said sauce en route to its destination.

"Dave, could you pass me the gravy please?"
"Of course, my dear."
Dave picks up the gravy boat, but pours some onto his plate first
"Oi, you cheeky fucker! Stop saucejacking and pass it here right now!"